I'm honestly just going by their lore kit. Ryoma isn't a demon slaying servant, but Oryou has divinity, so she should be able to hurt anything (theoretically), since, well, divinity. Servants hurting souls and stuff is a whole other debate that Im not good at, since I don't exactly know that much fate lore, as I've probably admitted several times somewhere. All in all, you may be right with the soul stuff, and I may be wrong, but my interpretation of it is that unless specified, heroic spirits don't hurt souls... unless they can eat em... because they aren't souls. We can see from the wiki page of the third true magic, "it is the temporary formation of a soul that does not require a real body, meaning that they are not truly brought back to life. In fact they are not considered proper Life but merely Ether in the form of past individuals.". I'm pretty sure Ether doesn't mean 'soul' here, although the previous line does make it pretty fuzzy.
Edit: Ryoma, here, is a counter guardian, which means that he is the actual soul of Sakamoto Ryoma. But we can see that, in fate, you either need an np to consume souls... or have a mage do it, like what Gil does to the orprhans in Kirei's basement (or does Kirei do that for Gil? I forget.). The fate wikis don't exactly have a section on damaging the soul as well (only destroying it), and the only thing is about anchoring the soul to the body. Vampirism has its own section, and states that vampires (or, well, demons) don't need the anchors because they can regenerate.
"
Vampirization - that is, making the flesh undead - is a way to prolong the soul after the death of the body; yet, the soul still decays. Vampires need to drink the blood of humans to maintain their body, and thus, their soul.
The core that binds the soul is the head - the brain. It is said that there are three parts required for a lifeform: the soul, the brain, and the body. The brain keeps the soul anchored to the body. The soul can reshape itself with the brain and the body. Usually, it is said that destroying either two out of these three is required to completely kill someone. In the case of a vampire, it seems the vampirization, which transforms the soul, can bypass the necessity of the brain to hold the soul: after a while, vampires can regenerate their body, and thus, their brain, to maintain the soul."
-from another type moon wiki
And about Ryoma's stats....
"Although Nobunaga regards
Sakomoto's Noble Phantasm in high regard, she states that Sakomoto himself is an especially weak
Heroic Spirit. Sakamoto agrees, adding that compared to her, almost every other Heroic Spirit aiding
Chaldea during the
Imperial Tokyo Singularity is third-rate at best." from type moon wiki's page on Ryoma. So, well, Ryoma is weak lol.